1. Field of the Invention
This invention concerns a liquid type of gas seal, in general. More specifically, it concerns an improved gas-seal type of check valve that is particularly useful in connection with a gas inerting system, such as one used on petroleum cargo tank ships and the like. The invention is an improvement upon a U-tube type of liquid seal that is known and that has operational problems.
2. Description of the Prior Art
In low pressure gas inerting systems, and particularity such systems applicable to petroleum cargo tank operations, it has been found that mechanical non-return valves are unsatisfactory because they could not seal "gas tight" at the pressures encountered. While liquid (usually water) seals are frequently used as positive non-return devices which operate like a common liquid trap, there has been difficulty with a method employed to positively prevent back flow while permitting free gas flow in the forward direction.
Specifically, there is a known deck seal designed by Howden Engineering Limited that employs a device which allows water to fill a U-trap from a reservoir. The ullage space of the reservoir is connected to the throat of a venturi that is placed in the inert gas delivery line. The object of that arrangement is to create a suction in the ullage space which will remove the sealing water from the U-tube whereupon the seal will operate "dry" so as to prevent the carry-over of sealing water into the cargo tank or tanks. Of course, any such carry-over of water is detrimental because of corrosion, as well as blocking of the inert gas mains in the system.
However, experience with the Howden type seal has indicated that it does not operate as intended. In the first place the venturi is not able to act, i.e., apply suction, without inert gas flow. And, such gas flow can not occur until the bottom of the U-tube has cleared. Furthermore, the necessary degree of suction is not created by the venturi at relatively low gas flow rates. Consequently, before sufficient flow rate has been reached there will be substantial carry-over of the liquid from the seal. It has been found that a Howden type of equipment will have substantial carry-over of the liquid persist up to 86% of system capacity.
Consequently, it is an object of this invention to provide an improvement over a Howden type of seal. The improvement creates suction on the liquid of the seal from the input of the pump or fan that causes inert gas flow in the system. Consequently, the liquid carry-over is substantially eliminated.
Another object of the invention concerns the use of a fail-safe arrangement. It ensures the connection of the space above the reservoir liquid to the down stream side of the U-tube seal under failure conditions. Consequently, it is effective to block any backward flow of gas and thus eliminates danger of any explosive gas backing into the inert gas source, where the environment includes possible explosive conditions as caused by high temperatures and the like.